The invention relates to a process for the lost foam casting, under low pressure, of metallic articles of aluminium alloy and constitutes an improvement to the process as described in French patent No. 2606688 filed on Nov. 17, 1986.
A person skilled in the art knows, for example from the teaching of U.S. Pat. No. 3 157 924, to use, for the casting of metals, patterns made of a foam of organic substance such as polystyrene which is immersed into a mould formed by dry sand containing no binder. Industrially, these patterns are generally coated with a film of refractory material intended to improve the quality of the castings. In such a process, the metal to be cast, which has previously been melted, is brought into contact with the pattern by means of a feed orifice and channels traversing the sand and gradually replaces said pattern by burning it and transforming it mainly into vapour which escapes between the grains of sand, hence the designation of the process as a lost foam casting process.
In comparison with conventional casting in a non-permanent mould, this technique obviates the prior manufacture, by compacting and agglomeration of powdered refractory materials, of rigid moulds connected in a fairly complicated manner to cores and allows easy recovery of the castings and simple recycling of the casting materials.
It is therefore simpler and more economical than the conventional technique. Furthermore, it offers greater freedom to the designers of castings with regard to the shape of said castings. This is why this technique has been found increasingly attractive from the industrial point of view. However, it is handicapped by several drawbacks, of which two result from conventional metallurgical mechanisms, that is:
the relative slowness of solidification which promotes the formation in the castings obtained of blowholes originating from the hydrogen dissolved in the liquid aluminium alloy
the relative weakness of the thermal gradients which promotes the formation of microshrinkage despite the presence of feeders.
With the object of avoiding such drawbacks, it has previously been proposed, in French patent No. 2606688, to apply to the mould, after filling and before the solidified fraction of the metal exceeds 40% by weight, an isostatic gas pressure having a maximum value of between 0.5 and 1.5 MPa.
Therefore, the process according to this prior French patent comprises the conventional stages of lost foam casting, that is:
employing a pattern of the article to be cast formed from a foam of organic substance coated with a film of refractory material;
immersing said pattern in a mould formed from dry sand without binder;
filling the mould with molten metal to burn said pattern, this filling being carried out through a feed orifice connecting the pattern to the exterior of the mould;
evacuating the vapour and liquid residues emitted by said pattern during the combustion thereof;
allowing, the molten metal to solidify to obtain the article.
As an improvement in the French process, when the mould is completely filled, that is when the metal has completely replaced the pattern and the majority of the vapour has been evacuated, a gas pressure is applied to the mould; this operation can be carried out by placing the mould in a chamber capable of withstanding the pressure and connected to a source of gas under pressure.
This operation can be carried out immediately after filling while the metal is still completely liquid, but it can also take place later providing that the fraction of solid dendrites formed during solidification in the mould does not exceed 40%, the pressure only having a negligible effect beyond this value.
The maximum value of the pressure applied in this French process is preferably between 0.5 and 1.5 MPa.
Under these conditions, it is found that the quality of the articles obtained is improved and this phenomenon is explained by the following mechanisms:
compaction of the blowholes, of which the volume is reduced in practice in the ratio of the absolute pressures prevailing during solidification. Thus, for example, when an absolute pressure of 1.1 MPa is applied whereas this pressure was previously atmospheric pressure, that is 0.1 MPa, this reduction takes place in a ratio of about 11;
better supply of the mould since the pressure exerted on the still liquid feeders forces said liquid through the network of dendrites formed at the beginning of solidification and hence quasi elimination of microshrinkage.
However, it has been found in certain cases that the application of a relative pressure higher than 0.5 MPa led to the appearance of particular defects known as "spongy shrink holes" which is explained as follows: if the cast alloy has a relatively great solidification range, a pasty zone develops within the article; moreover, if the distance between the feeder and the location where the shrink hole occurs is great relative to the thickness of the article, the pasty zone creates a significant loss of charge over the metal supply to the mould with the result that the feeder itself cannot play its part under the influence of the external pressure, that is to say cannot sufficiently rapidly supply the shrink holes which are being formed.
As the solidification range is relatively great, the "skin" of the article (portion located at the metal/sand interface) is brittle for much longer and the external pressure exerted by the application of the gas on the sand therefore depresses this skin toward the interior of the article, allowing a fraction of gas to infiltrate between the dendrites toward the shrink holes and thus creating a so-called "spongy" shrink hole which is as harmful as the conventional shrink hole with regard to the obtaining of good mechanical properties.
Consequently, if articles are to be cast from an aluminium alloy having a relatively great solidification range and if the geometry of said articles leads to a relatively great distance between the feeder and the shrink hole zone known as the critical zone relative to their thickness, it is desirable to avoid these phenomena by eliminating the application of a pressure for example. However, it would be a pity to forgo this technique of casting under pressure which, moreover, leads to a considerable improvement in the quality of the castings.